The PCP guns with a moderator are quiet, but without one they can be very loud. Even some of the magnum spring guns that break the sound barrier with light pellets can be loud as well. In general you'll find the lower to mid powered spring guns to be quieter than the magnum guns.
Here's a few links where they tested the airgun decibels.
https://www.riflezone.com/best-quiet-air-rifles-comparison/http://www.straightshooters.com/decibel-ranking-of-airguns.html
